1. Field of the Invention
This invention relates generally to printing color images by construction from individual ink drops deposited in pixel arrays; and more particularly to creation of such images by inkier operation on a printing medium whose hue or saturation response, or both, depart from the responses of ordinary paper. Certain aspects of our invention are particularly pertinent to binary printing operation, and even more particularly relevant in the context of three-binary-bit color reproduction.
2. Prior Art
Printing with primaries and secondaries
In this well-known technology, color printing is controlled by programmed digital microprocessors. In each apparatus one or more processors operate pens loaded with ink of three so-called "primary" colors--most commonly cyan, magenta and yellow.
Printing any of these three colors is accomplished simply by depositing ink of the desired color. So-called "secondary" colors--ordinarily green, blue and red--are printed by combinations of the primaries.
More specifically, printing of any one of the secondaries (green, for example) is done by overprinting two primaries (traditionally equal quantities of cyan and yellow, for the same example). The secondary is created by the subtractive process of spectral absorption.
Commonly, still another pen is loaded with black ink to provide black and gray effects. This aspect of the printing process may not be of direct interest in regard to the present invention and so will receive little mention in the remainder of this document.
Binary operation, and number of bits in the system
By "binary" we refer to the well-known concept of either having or not having a fixed unitary quantity of something. In this context it means either printing or not printing in each pixel location a particular primary or secondary color--in an essentially unvarying amount.
This concept is sometimes summarized with the phrase "go/no-go". In other words, either the system enters a "go" mode of operation--with respect to a particular primary or secondary color--in which it may be said to "go ahead and print" some specific, fixed quantity of that particular color; or it enters a "no-go" mode in which it does not print that color at all.
Not all pixel-array printing systems operate in this way. To the contrary, some systems use a multiple-level approach, in which many different quantities of ink (of one or several colors) can be deposited at each pixel.
The point of those other systems is to establish a multiple-level saturation scale, or--viewed in a mono-chrome sense with respect to each color independently--a multiple-gray-level scale, which can be implemented for each color at each pixel. A multilevel system must be implemented by multilevel data processing in the microprocessor that controls the pens.
In an ordinary programmed digital microprocessor, such multiple-level data are necessarily represented in multiple-bit form. Although each bit used to express the quantity of ink of course remains binary (go or no-go), the overall quantity itself is now truly susceptible to quantification--that is, expression in gradations on a scale.
In a multilevel system the multiple-bit information processing and the multiple-ink-drop physical control required are both very demanding. Furthermore these provisions are required in each channel--in other words, required at least for each of the primaries that are employed by the system, and in some operational philosophies each of the secondaries too.
A system using, for example, five-bit levels to distinguish 2.sup..zeta. =32 saturation gradations in each of three different channels (primary-color inks) is overall at least a seven-bit system. At least two bits must be used to identify which ink is to be controlled by the five "level" bits.
For simplicity of operation it may be preferred instead to use three bits for identification--thus allowing the system to work in the six primaries and secondaries (plus black and white) rather than three (or four) inks. Thus an optimum five-bit-level (thirty-two-gradation) system may be an eight-bit system overall.
Such systems are capable, at least in principle, of greater hue and saturation fidelity, particularly for very fine features of images. They require, however, either longer operating times or microprocessors and system-bus architectures capable of handling a greater number of bits in parallel--or both.
Of course such systems are available, and new ones are readily designed for the purpose; and they are less costly nowadays than previously. They remain, however, expensive.
Even these data-processing provisions are not the end of the matter, for fine physical control of inking is needed to implement such a system reliably. Such physical provisions, especially under varying environmental conditions, remain very touchy or temperamental.
Accordingly for such performance a price must be paid. Ordinarily it is paid in terms of printing speed, drying time, apparatus complexity and cost, price to the end-user, sometimes uniformity of results, etc.
One known system that employs a thirty-two-increment scale--that is to say, up to thirty-two small drops per pixel--is known as the Hertz technology. In that system a piezoelectrical crystal vibrates a droplet of ink, while the system either does or does not electrically charge the droplet.
Applying a charge enables the droplet to escape from the vibrator with momentum imparted by the vibration. Thus this particular system, in addition to being a multilevel, multibit system does not operate by thermal-inkier technology; moreover, the drops are used to express complex colors, rather than primaries or secondaries.
Our invention is addressed to refinements that very closely approximate such high-color-fidelity performance--but in a device that is vastly simpler and more functionally efficient, and therefore economical. Thus our invention aims to solve a problem that in the past has obstructed achievement of high-quality but inexpensive color reproduction.
Another technology which superficially may seem to be related to ours in that it deals with depositing multiple ink "dots" or drops--notably drops of different density--within a given "picture element" is U.S. Pat. No. 4,672,432, of Sakurada. Each drop is either full-density of one-quarter of full-density.
In Sakurada moreover, each "picture element" is not a pixel but rather a three-by-three matrix of pixels; and the multiple dots are never overprinted but rather only adjacently printed in adjacent pixels of each "picture element". This is shown in the Sakurada patent particularly by the unavailability of patterns for tone levels 31, 34 and 35: if overprinting were permitted in that system, patterns would be available for those level.
Hence Sakurada's total inking in each nine-pixel (three-by-three) matrix is always at most equal to--and usually much less than--nine times the amount of ink in one full-density ink drop. Thus on average the inking is some fraction f.ltoreq.1 of a single full-density dot.
Sakurada's system provides thirty-three different tone levels for each color unit, and is thus distinctly a multilevel system. He also, however, permits mixing inks within each three-by-three matrix for the purpose of creating complex colors within each matrix--and so is at the same time a multihue system; such a system is a form of, or a substitute for, the so-called "rendition" process discussed in the following subsection.
U.S. Pat. No. 5,012,257 of Lowe et al. describes a system for inkjet printing on paper, in which two-by-two groups called "superpixels" are used for preestablishing certain color combinations, or complex colors, that can be called for as a unit. Unlike Sakurada, Lowe uses ink drops of all the same density and size; due to this fact, as well as the fact that each of Lowe's pixel groupings has only 4/9 as many pixels as Sakurada's, Lowe's system provides many times fewer levels of overall inking density, but it is nevertheless a multiple-level system.
In Lowe, at most two pixels in each two-by-two super-pixel are printed--which is to say that at least two are blank, or white (for white paper). Also Lows uses no more than two drops per pixel, or three drops per two-by-two superpixel. Therefore in Lows the number of drops per pixel on average is always less than one, and in fact is at most three-quarters.
The Sakurada and Lows inventions thus provide what might be characterized as a hue-on-demand system. Such a system may for instance work out the hue desired at a given pixel, and then select the closest available hue--provided by the multiple-hue selections of the three-by-three or two-by-two pixel grouping.
The hue selection is thus performed on a pixel-group basis, or in other words for an entire pixel group as the basic unit of color, rather than for each pixel individually. As already noted above in relation to Sakurada, this is a substitute for--or form of--rendition.
Still another technology of interest, but of even more-superficial similarity, is disclosed in U.S. Pat. No. 4,528,576 of Koumura. That document contains diagrams showing three colors seemingly stacked or overprinted. Careful reading, however, suggests that those diagrams are merely figurative representations of the sequence in which colors--any two colors--are put down and layered, if and when they are put down; it is not believed that Koumura intends to convey that all three colors ever would actually be put down on a single pixel.
Mapping primary and secondary colors to inks
The present document focuses upon an operational stage that is between (1) the selection of a primary or secondary color, and (2) the physical printing process. In other words this invention addresses the stage of determining which ink to discharge (or to put it another way which pen should fire)--and how much of that ink to discharge, and where to discharge it--to implement a specific primary or secondary color at a specific location.
More succinctly, we are mapping primary or secondary colors to primary inks, on a small-scale (single pixel or nearly so) basis.
To assist in recognizing differences between certain aspects of our invention and the prior art, however, we will briefly digress to describe a function known as rendition.
Direct representation vs. rendition
In printers of the type under consideration in this document, microprocessor programs implement the ink applications needed to produce a desired color effect at each part of an image. The decision to print a particular primary or secondary color at a particular pixel is made in one of two ways:
(a) sometimes the decision follows directly from the desired or so-called "input" color of a desired image at that pixel location--in other words, sometimes a user calls directly for a primary or secondary color there; and PA1 (b) sometimes the selection of a particular primary or secondary is the result of a complicated resolution of a complex color at each pixel location into component primary and secondary fractions--and then in effect implementing those fractional representations by a real distribution, in a progressive assignment procedure called "rendition" and sometimes employing a propagating process called "error diffusion". PA1 defining a superpixel that consists of a plurality of pixels associated with said particular pixel, and PA1 applying to each of the pixels in that superpixel, on the given printing medium, a respective number of drops of ink predetermined for the desired primary or secondary in conjunction with the given printing medium. PA1 each of two opposite corners of the cluster receive one drop of ink of a first primary color and one drop of ink of a second primary color, and PA1 each of two other opposite corners of the cluster receive one drop of ink of the first primary color and two drops of ink of the same second primary color.
The second type of decision-making is used to produce, for example, a recognized color blend such as chartreuse, or an undefined color needed to match the appearance of a scanned photograph. Details of various approaches to rendition and error diffusion which lead to such selection of a primary or secondary for printing--as well as the physical printing process itself--appear in the related patent documents that are identified above and incorporated into this document by reference.
The term "rendition" is now used in the color industry to mean a microprocessor-controlled process that operates in a semiintegrated way upon relatively broad areas of a pixel array (in contrast to the small-scale mapping that is of interest in our invention), to establish which primary or secondary colors will be used at each pixel location to create a desired or "input" image. This rendition process in effect represents a transform that is performed upon an entire desired image to create its representation or equivalent as an entire pixel array.
Most typically the rendition process actually executed by a programmed microprocessor may become particularly elaborate when the desired hues of the input image are relatively subtle washes. Such hues are just slightly removed from primaries and secondaries--rather than midway between a primary and a secondary, or actually equal to a primary or secondary.
In such instances the rendition program labors continuously to inject just the right amounts of the available primary and secondary colors needed to represent the tinges of minor components that create such subtle effects--and to make these minor-element injections in a reasonably well-distributed and consistent way, to avoid creating a perception of either mottled or erratic color artifacts.
Various algorithms are used to accomplish this. One group of such protocols creates a sort of continuous-dilution phenomenon in the pixel array. What is continuously diluted is color itself.
In these processes the program finds the best single primary--or secondary-color approximation to a desired color at each given pixel--and then determines what the resulting error is there. Then in subsequent (usually nearby) pixels the program attempts to compensate for that error while at the same time expressing the colors that are desired in those (subsequent) pixels. It is in keeping with the continuous-color-dilution character of these algorithms that they are known by the apt term "error diffusion".
In the prior art, such large-scale multipixel integration of color representation has been employed only for representing complex colors, or for automatically matching arbitrarily selected colors in input images--even though in operation these sometimes may turn out to be primaries or secondaries--and for dealing with errors already accumulated along the way in the representation process.
Alternative to this large-scale multipixel rendition process are smaller-scale plural--or multiple-pixel arrays such as used by Sakurada or Lowe to provide hues efficiently--in terms of computational time--but perhaps not as accurately for complex colors as the rendition process.
Lowe's superpixels permit selection of hues that are primary colors, such as cyan, magenta and yellow, and also other hues that are individual secondaries such as blue, green and red--but these six hues are only half of a total of twelve of which the system is capable, and in addition three double-intensity forms of the primaries are also provided. Thus the Lowe system is not in essence a postrendition process for binary expression of preestablished individual primaries and secondaries exclusively.
Media effects
In color printing with thermal-inkjet pens, inkdrop volume and colorant concentration for each of the three primary inks are usually designed to give the correct spot size and color saturation on the most commonly used printing medium--namely, paper. When these same pens are used to print on other media such as transparency film or glossy media, however, the spot size and color saturation may not be optimum.
For example, another printer commercially available from the Hewlett Packard Company under the model designation PaintJet.RTM., when used to print on paper, ejects one drop per pixel for primary colors (cyan, magenta and yellow) and two for secondaries (green, blue and red). In other words, the ratio of quantity of secondary to quantity of primary is substantially 2:1. We say "substantially" because as will be understood ink drops from different pens are not necessarily identical in volume, and as a practical matter need not be; in any event, to put the same statement in a less general way, the number of drops of secondary ink to number of drops of primary ink stands in the ratio 2:1.
These treatments produce good hue and chroma, as well as satisfactory appearance on a lightness-darkness scale, on paper. When used on transparency film, however, these same treatments produce primary colors that appear under-saturated, or in familiar terms "washed out".
To improve color saturation of colors printed on transparency film, the PaintJet.RTM. system operates in a special transparency-printing mode which applies two drops per pixel for both primary and secondary colors--in other words, here the ratio is 2:2. Related techniques for satisfactorily implementing such a strategy are discussed in U.S. Pat. No. 4,943,813 to Palmer and Morris.
The teachings of that patent are representative of modern methods for inkjet printing on transparency film. Those teachings resolve the problem of multiple dot over-printing--and closely adjacent printing--by providing a time delay between successive passes to reduce dripping and improve inking uniformity.
Application of two drops per pixel for all primaries and secondaries, a 2:2 ratio, for transparency film is far superior to the 2:1 relationship that is optimum when used on paper. Nevertheless it does leave some room for refinement: we have noticed that to most viewers both red and green appear slightly undersaturated if printed in this way on transparency film.
This is especially significant in regard to modern printer products because--among other well-known considerations--red is if anything the most important single commercial color, and at the same time one for which casual observers tend to have a particularly critical eye.
Another, more serious, problem arises in color printing on other glossy media. In particular some popular printing materials have a glossy coating, somewhat like transparency stock, but also an opaque backing.
Such a printing medium is commercially important because of the added visual snappiness or flashiness which it can lend to a finished product. Exemplary products include commercial announcements, posters, and covers for comb-bound books. In this case the 2:2 ratio has been found satisfactory for green and blue, but red appears badly undersaturated.
As can now be seen, important aspects of the technology which is used in the field of the invention are amenable to useful refinement, which the prior art has failed to provide.